Gas Laws in Action

Introduction

In this lab we investigated two of the ideal gas laws: Boyle’s Law and Charles’s Law. We subjected a balloon to greatly reduced pressure and used volume measurements to find that we had reduced the pressure from 1.0 atm to 0.20 atm. We also cooled and warmed a balloon and measured the volume at each temperature. We compared the volume measured at each temperature to the volume predicted by Charles’s Law and found that the values were close but not close enough. For instance, using the 274 K volume of 230 cm3 we found that the 310 K volume should be 260 cm3. This is 12 cm3 bigger than the experimental value of 248 cm3. We attribute the difference to the fact that the diameter of the balloons was very hard to measure accurately.

Materials

  • 500 mL filter flask (vacuum flask)
  • no-hole stopper to fit
  • safety goggles
  • cm ruler
  • vacuum tubing
  • sink aspirator
  • ring stand
  • clamp
  • 2 small balloons
  • 400 mL beaker
  • hot water bath
  • lab notebook & pen
  • ice & water
  • thermometer(s)

Procedure

This lab had two parts. In the first part my lab partner and I investigated Boyle’s Law. In the second part we looked at Charles’s Law.

For the investigation of Boyle’s Law we inflated a small balloon to a small size. It had a height of 5.2 cm and a diameter of 3.1 cm when we tied it off. According to the formula for the volume of a cylinder (V = r2h) the volume was approximately 39.2 cm3. We placed the balloon inside a 500 mL vacuum flask and put the stopper on tightly. The flask had already been connected to a sink aspirator, a device that draws air and can be used to create a partial vacuum inside the flask. We turned on the faucet full blast to start the aspirator and waited for it to work. After about 5 minutes (this wasn’t timed) we noticed that the balloon inside the flask had become quite a bit larger. By the time we decided to measure its size the diameter of the now spherically shaped balloon was 7.2 cm. This is not very exact because we could not measure the balloon directly but had to make a measurement from the outside of the flask. Treating the balloon as a sphere (V = (4/3)r3) it had a volume of 195 cm3. We loosened the stopper of the flask with some effort and saw the balloon instantly shrink to its former size and shape accompanied by a soft whooshing noise.

Next, we obtained another balloon that had already been inflated to a roughly spherical shape with a diameter of 7.7 cm. Again, treating the balloon as a sphere, the volume was 239 cm3. We cooled this balloon off in an ice-water bath with a temperature of 1°C (274 K). I held it under the surface of the ice-water using a small beaker and occasionally stirred the bath with the thermometer. Once my lab partner and I agreed that it was probably the same temperature as the bath I pulled the balloon out and measured its diameter. It was difficult to measure the diameter in a way consistent with how I had done it when the balloon was at room temperature but I found that d = 7.6 cm. This means a volume of about 230 cm3, assuming the balloon is spherical. This isn’t much less than the volume at room temperature but the diameter I measured may not be perfectly accurate.

My lab partner now put the balloon into a warm-water bath heated on an electric heater. Once he thought it was warmed up to the same temperature as the bath I measured the temperature: 37°C (310 K). I measured the diameter of the warmed-up balloon as soon as possible after we took it out of the bath and I found that it was now 7.8 cm. Again, this is not a lot bigger than the room temperature measurement or even the cold-water measurement, but I don’t think I can guarantee the accuracy of the number. Assuming that it is correct a diameter of 7.8 cm gives a volume of 248 cm3 (for a spherical balloon).

Results

The results for this lab can be summarized with the data in the following two tables:

Boyle’s Law:

V1 / V2 / P1 / P2
39.2 cm3 / 195 cm3 / 1.0 atm / 0.20 atm

Charles’s Law:

Vcold / Tcold / Vroom / Troom / Vhot / Thot
230 cm3 / 274 K / 239 cm3 / 298 K / 248 cm3 / 310 K

For the Boyle’s Law table I have calculated the pressure inside the flask assuming that the pressure in the room was exactly 1.0 atm. I used the formula: P1V1 = P2V2 to do this. Even given some fairly large errors in the exact volume of the balloon at high and low pressure it is clear that the pressure was very low when the balloon swelled up to that relatively enormous size.

The Charles’s Law table shows just the experimentally measured temperatures and volumes. Using Charles’s Law (V1/T1 = V2/T2) to predict the cold temperature volume using the hot temperature volume and temperature I find that it should be about 219 cm3. The calculated value is smaller than the value I measured and might mean that there was an error in my measurement of the diameter of the balloon.

When I use the cold temperature volume and temperature measurements I find that the hot balloon should have a volume of 260 cm3. This is 12 cm3 bigger than what I found but the difference is probably due to the near impossibility of measuring a good diameter for the balloons.